Method for producing low carbon steel with exceptionally high drawability

ABSTRACT

1350* F. FOR 12 TO 30 HOURS. R$ VALUES GREATER THAN 2.0 MAY BE OBTAINED BY FINISH HOT ROLLING WITHIN A NARROW TEMPERATURE RANGE (JUST ABOVE THE A3) AND EMPLOYING LESS THAN 0.07/MN.   A SHEET STEEL WITH EXCEPTIONALLY HIGH DRAWABILITY (R VALUE GREATER THAN 1.5) AND HIGH YIELD STRENGTH, IS PRODUCED BY ADJUSTING THE MELT TO LESS THAN 0.15% MN, 0.03 TO 0.1% C, 0.004 TO 0.03% S AND LESS THAN 150 P.P.M. OXYGEN. THE SLAB CAST FROM THE ABOVE MELT IS THEN HOT ROLLED, ONLY ABOVE THE A3 TEMPERATURE, COLD REDUCED FROM 60 TO 80% AND SOAKED AT TEMPERATURES OF 1200* F. TO

Jan. 9, 1973 s. R. GOODMAN E AL 3,709,744

METHOD FOR PRODUCING LOW CARBON STEEL WITH EXCEPTIONALLY HIGH DRAWABILITY Filed Feb. 27. 1970 I500 I600 I700 I800 'F I I I l l l l l 800 850 900 950 I000 '6 HOT-ROLLING TEMPERATURE FIG. 2.

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A r rarney United States Patent 3,709,744 METHOD FOR PRODUCING LOW CARBON STEEL WITH EXCEPTIONALLY HIGH DRAWABILITY Stephen R. Goodman, Monroeville, and Hsun Hu, Franklin Township, Westmoreland County, Pa., assignors to United States Steel Corporation, Pittsburgh, Pa.

Filed Feb. 27, 1970, Ser. No. 15,018 Int. Cl. C22c 39/30; C21d 9/48, 7/14 US. Cl. 148-12 10 Claims ABSTRACT OF THE DISCLOSURE A sheet steel with exceptionally high drawability (R value greater than 1.5) and high yield strength, is produced by adjusting the melt to less than 0.15% Mn, 0.03 to 0.1% C, 0.004 to 0.03% S and less than 150 p.p.m. oxygen. The slab cast from the above melt is then hot rolled, only above the A temperature, cold reduced from 60 to 80% and soaked at temperatures of 1200 F. to 1350 F. for 12 to 30 hours. R values greater than 2.0 may be obtained by finish hot rolling within a narrow temperature range (just above the A and employing less than 0.07/ Mn.

BACKGROUND OF THE INVENTION This invention is directed to a method of producing sheet steel of improved deep-drawing characteristics by judicious control of both the compositional and processing variables.

At present, two basic types of low-carbon sheet steels are used commercially for deep drawing. These are the low-carbon rimmed steels and the aluminum-killed steels. Low-carbon rimmed steels are more economical and have better surface characteristics than aluminum-killed steels; however, they do not have the very high drawing properties of aluminum-killed steels which are necessary for more extreme draws. Other elements, particularly titanium, having a high afiinity to oxygen, are also sometimes used, alone or in conjunction with conventional deoxidizers, as aluminum, in the production of killed steels. Aluminum-titanium or aluminum/titanium-killed steels, however, although superior in deep-drawing properties, do not appear to be practical for continuous casting. These steels are also expensive due to high cost of the deoxidizer additions and to the low yield from ingots and high conditioning costs. It is, therefore, desirable to develop a new deep-drawing steel, which is relatively inexpensive and yet suitable for production by either conventional or continuous casting techniques.

The drawability of sheet material can be evaluated by simple tension tests. When a strip specimen is pulled to a greater length, its width and thickness are decreased. The plastic strain ratio can serve as an indication of the degree of mechanical anistropy of the material. This ratio is referred to as the R value and is defined as the ratio of percent change in width (e,,, the width strain) to the percent change in thickness (e,, the thickness strain), i.e.

where W and L are the width and length, respectively, of the gauge section, and the subscripts i and f refer to the initial and final measurements (before and after straining) of these dimensions. This expression is based on the assumption that the volume of the gauge section remains constant during testing and it eliminates the direct measurement of the thickness which owing to its small value in a sheet material yields less accurate results.

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The R value is, therefore, a useful parameter for indicating the degree of mechanical anisotropy of a given material. For an isotropic sheet, the R value is equal to one. If R is less than one, the sheet thins unduly and is, therefore, undesirable for drawing operations. For very deep draws, it is preferable to have R values equal to or greater than about 1.5, preferably greater than 1.7.

To obtain an average R value, tensile tests are conducted on several specimens taken at various angles, usually at 0, 45 and to the rolling direction. An average R value of the sheet can then be obtained as follows:

The difference among the individual R values indicates the earing tendency of the sheet during drawing. A uniform value of R is also highly desirable. The variation of R in the plane of the sheet is sometimes designated as AR, which is defined as:

Although AR=0 does not necessarily indicate an absence of earing, it is useful in describing the planar anistropy of the sheet.

In recent years the drawing properties of steel sheets have been correlated with crystallographic textures. Good drawability and high R values are associated with the cube-on-corner texture, or (111) planes parallel to the plane of the sheet. The next most favorable orientation for good forming properties is the (112). Lower R values are associated with both the cube-on-face texture and the cube-on-edge texture. Thus for good drawing properties the amount of the less favorable textures should be minimized. For optimum drawability the ideal textures consist of random orientations of (111) planes in the plane of the sheet, i.e., a (111) fiber texture with the sheet plane normal as the fiber axis.

The crystallographic texture of a specimen is normally determined by the construction of complete pole figures from X-ray intensity measurements; however, for detection of small variation in the texture, a direct comparison of two pole figures cannot reveal the detailed dilferences quantitatively. Accordingly, it has been found best to measure the integrated peak intensities of several reflections from the plane of the sheet and express them in units of corresponding peak intensities of a random specimen. The numerical values of these relative intensities so obtained are directly proportional to the pole densities of a specific plane lying parallel to the plane of the sheet. Since the drawability of a sheet depends on the relative population of specific crystallographic planes in the plane of the sheet, this technique is very useful. The intensities of five diflerent reflections, i.e. (110), (200), (112), (310) and (222) are measured. The intensity of the (222) reflection which is the second order reflection of the (111) therefore represents the amount of (111) texture. Similarly, the intensity of the (200) reflection represents the amount of the (100) texture, respectively. The correlation between R values and texture has been found to be very consistent in actual test results.

It is known that a steel sheet having an R value above about 1.20 exhibits superior drawability, and prior art deep-drawing sheets have generally possessed maximum R values of about the latter value, for example, as disclosed in E. H. Mayer et al. US. Pat. No. 3,244,565. Higher R values are known, for example, as disclosed in US. Pat. No. 3,404,047 of the present inventors. Such high R values have been heretofore associated with very low carbon contents, as obtained, for example, by prolonged decarburizing annealing treatments, as described in the above patents.

The plastic strain hardening exponent, n is a second fundamental factor in deep drawability, in conjunction wtih R values. The exponent n, is determined experimentally as the slope of the plot of log true stress versus log true strain in the region of uniform strain. High n values, like high R values, are favorable to improved drawability, although the n value is more markedly dependent upon microstructural factors than upon crystallographic texture. For low-carbon steels, n usually varies from 0.20 to about 0.30 in drawing quality steels.

It is, therefore, an object of this invention to produce a steel sheet having a crystallographic texture with a predominance of (111) and (112) planes in the plane of the sheet.

It is another object to provide a steel composition which, when processed by the method of this invention, will yield extremely high E values, i.e. values greater than about 1.5, and preferably greater than 1.7, together with good n values.

A further object of this invention is to provide a composition of only moderately low carbon content which will yield a combination of high E values and low AR values without sacrificing yield strength and without the necessity of subjecting sheets thereof to expensive de- Composition, wt. percent Broad Preferred Element:

Carbon 0.03 to 0.10. 0.03 to 0.076. .15 0.005 to 0.07.

Oxygen. .I 0.015 max 0.01 max. Iron Balance, except for incidental steelmaking impurities 'lhe upper limit of 0.02% is set by the need to control hot-shortness. If this tendency is reduced by addition of other elements (e.g. Al or T1), then somewhat higher values are desirable for minimizlng planar anisotropy.

The criticality of the combined composition and processing steps will become apparent on reference to the following description and figures, wherein:

FIG. 1 is a graph depicting how the R value obtainable is a function of hot rolling temperature.

FIG. 2 is a graph relating manganese content of the steel compositions of this invention with obtainable R values for sheets processed as herein described.

TABLE I.COMPOSITION OF STEELS TESTED Weight percent Mn S Si P Cu Ni 01 N Alt O carburization treatment.

Two compositions (Table I, samples I and II) were Another object of this invention is to provide a process 50 hot rolled to a final thickness of 0.100 in. in six nearly for producing sheets of the above compositions so that maximum E value potential may be achieved.

This invention is based on the discovery that a number of compositional and processing variables are critical for providing maximum E values. To maintain the necessary high yield strengths, the carbon content of deep-drawing steels is generally in the range of .03 to .1%. Thus, if the melt of such a steel is adjusted so that the Mn content is kept below about 0.15% and the oxygen content below about .015 the subsequent critical mechanical and heat treatments will yield a steel with the desirable high E values. It is essential to the instant invention that the cast slab, having a composition within the foregoing range, be hot rolled at a temperature sufiiciently high to ensure that the final hot rolling pass be made above the temperature at which proeutectoid ferrite will form. After such hot rolling, the steel must be cold reduced beween 60% to 80% and annealed by soaking for at least twelve hours at a temperature of 1200-1350 F. Thus, it has now been found that a steel composition having critically limited amounts of carbon, manganese and oxygen, together with controlled maximum quantities of sulfur, phosphorus and silicon, if processed as herein described, is productive of deep-drawing steel sheets having exceptionally high E values, over 1.5, and of good n values.

equal reduction passes. After each pass, the plate was reheated for two minutes to restore temperature; the direction of rolling being reversed for each pass. The strips were air cooled, cold rolled 70% and annealed in dry H by heating to 1310 F. (at 36 F./hr.) and holding at temperature for 20 hours (i.e., the method of US. Pat. No. 3,404,047). The effect of rolling temperature on E value is shown by FIG. 1. While the effect of rolling temperature is dramatic for both steels, it is even more pronounced for the low Mn, low 0 steel (11). The maximum E is reached by rolling just above the A temperature. At higher rolling temperatures, this value decreases, but not as rapidly as at lower temperatures.

The importance of manganese content is depicted below (Table II) and in FIG. 2. A series of vacuum melted steels (Table I, samples A-G) were hot rolled at 1700 F. in 6 passes and cold rolled and annealed for 20 hours at 1300 F. in a 6% H 94% N atmosphere. To attain an E value above 1.5, the steel should contain less than about .15% Mn. In commercial practice, it will be very diflicult to maintain as precise a control on hot rolling temperature as in the reported tests; therefore, in order to ensure the attainment of an E value above 1.5, it will be generally necessary to employ a much lower Mn content, i.e. from about 0.005% to about 0.07% Mn.

TABLE II [Effect of Manganese Content on Mechanical Properties of Low-Carbon Steel Sheet] Annealed Avg. ASTM Percent Y. grain n R R R45 Ran AR (p.s.1) size The effect of sulfur content on texture development and E value is shown in Table III. The hot-rolling, coldrolling and annealing was substantially as described above.

TABLE III [Efiect of Sulfur Content on Mechanical Properties of Low-Carbon Steel Sheet] Annealed Avg. ASIM Percent Y.S. grain Ste S R R0 R Ran AR (p.s.i.) size The results show that while low sulfur content is not essential for high It values, it is however, possible, (in a low Mn steel) by judicious control of the sulfur content to produce a steel with a minimum amount of planar anisotropy, i.e., lower AR values. Pole figures, (not shown) constructed from X-ray intensity measurements, have shown that this interchange of '11 values at 0 and 45 is related to the following crystallographic texture changes:

(1) A somewhat more uniform (111) fiber texture in the plane of the sheet and an increased (112) component, both of which increase R at 45.

(2) An increase of the (200) component which decreases R for the 0 direction.

Thus, a range of 0.01% to about 0.03 sulfur is desirable, both for minimizing planar anisotropy and increasing the yield strength (e.g., see also steels N through Q). The upper limit being determined by the tendency to hot shortness with such low Mn steels. Of course, if addition elements such as Al or Ti are used to diminish this problem, then somewhat higher values of sulfur may be employed.

Another composition variable found critical for the purpose of this invention is the oxygen content, which should be low to minimize or prevent hot shortness of the steel during hot rolling. It may be seen from Table IV that when the oxygen content in the steel reached a lavel of 250-300 p.p.m., there was a detrimental effect on the development of a steel with high If values.

TABLE IV [Plastic Strain Ratio Values of a LowCarbon, Low-11in Steel at 0xygen Levels of 250350 p.p.m. and Variable Sui-fur] This detrimental effect may be due to the fact that the oxygen which is in the form of oxide and/or silicate inclusions, will act as preferred nucleation sites for recrystallization. To attain the desired i values of this invention it is therefore necessary to reduce the oxygen content of the steel to below about ppm; generallyby vacuum carbon deoxidation. 1 While coiling temperature does not appear to be critical in the usual temperature range of 1000 to 1200 F., it is best not to exceed 1150" F. for optimum properties. To simulate coiling procedure, steel W was hot rolled at 1710" F. in six passes to 0.105 in. Four separate sections were treated in the following manner, before being cold rolled 70% and annealed as described above.

(a) Air cooled to ambient temperature,

(b) Air cooled to 1000 F., held 4 hrs., then air cooled,

(c) Air cooled to 1100 F., held 4 hrs, then air cooled,

((1) Air cooled to 1200 F., held 4 hrs, then air cooled.

TABLE V [Effect of Simulated Cloilingtlent aerature on Tensile Test Data of S ec Annealed Y .8 grain R R0 R4 R 0 A R (p.s.i.) size [Plastic Strain Hardening Exponent, n, and Ductility of Steel 0 (above) (Average of Two Measurements) As seen from the table above, a simulated cooling temperature of 1200 F. produced a somewhat lower it value and higher AR.

From the above data it is clear that a low-carbon steel sheet with fi values greater than 1.7 and normal T1 values can be produced by operating within the prescribed composition and processing limits of this invention. The exceptional qualities of this steel are directly related to the crystallographic texture, primarily a (111) fiber texture in the plane of the sheet, which is developed during the final box anneal. The composition of the steel sheet lends itself to manufacture by either normal ingot practice or by continuous casting.

The product of this invention is therefore produced in the following manner. The heat is melted to the prescribed composition. Vacuum carbon deoxidation of the melt is employed to reduce the oxygen content of the steel to 150 ppm. or less so that the resulting steel slab (produced by either convention ingot practice or by continuous casting) has a composition within the prescribed range. The slabs are hot rolled (reheated if needed) at a sufiiciently high temperature (-2250 F.) to ensure that the final hot rolling pass is made at a temperature above that at which proeutectoid ferrite will form. The steel is then coiled at about 1000 F.-1150 F. and cooled in air to room temperature. Hot mill scale is removed and the steel is cold reduced 60-80% to final gauge. The desired maximum E values will not be as readily obtained with cold reduction outside of this range, and it is therefore preferable to adjust the hot rolled thickness, so that a cold reduction of about 70% will provide the final gauge. The strip is then open wound and box annealed by conventional practice, i.e. slow heating to soak temperature and a soak of approximately 1300" F. for a minimum of 12 hours, with about 20 hours being preferred to obtain the desired texture and high E values disclosed by this invention.

We claim:

1. A method for producing a sheet steel of exceptionally high drawability which comprises (a) adjusting the composition of a steel melt so that it consists essentially of manganese .005 to .15%, car- 7 bon .03 to .1%, sulfur .004 to .03% and less than .015% oxygen, balance iron;

(b) hot rolling a slab produced from said melt at a temperature sufficiently high to prevent the formation of proeutectoid ferrite;

(c) cold reducing the resulting sheet to from 60 to 80%; and

(d) soaking the cold reduced sheet at a temperature of 1200 F. to 1350 F. for a period of 12 to 30 hours, so as to impart a crystallographic texture with a predominance of (111) and (112) planes in the plane of the sheet.

2. The method of claim 1, wherein the planar anisotropy of the sheet is minimized by adjusting sulfur content of the melt to above .01%.

3. The method of claim 1, wherein the manganese content is adjusted to below .07% to ensure attainment of an E value of greater than 1.5, under commercial hot rolling practice.

4. The method of claim 1, wherein the adjustment of the oxygen content in step (a) is accomplished by vacuum carbon deoxidation.

5. The method of claim 4, wherein the final pass in said hot rolling procedure is conducted within the range of 1700-1830 F.

6. The method of claim 5, wherein said cold reduction is about 70% and said soak temperature is about 1300 B, so as to maximize the E value of the sheet.

7. The method of claim 5, wherein said melt is continuously cast to form said slab.

8. The method of claim 1, wherein the composition of the melt is adjusted so that it consists essentially of, by weight percent carbon-0.03 to 0.075 manganese-0.005 to 0.07 silicon0.06 max. sulfur0.01 to 0.02

phosphorus-0.01 max.

oxygen-0.01 max.

iron-balance, except for incidental steelmaking impurities.

9. A sheet steel composition with an F value greater than 1.7 and a yield strength above 30,000 p.s.i., consisting essentially of, by weight percent carbon--0.03 to 0.10 manganese-0.005 to 0.07 sulfur0.004 to 0.03 oxygen-0.015 max. iron-balance, except for incidental steelmaking impurities.

10. -A composition in accord with claim 9, consisting essentially of carbon0.03 to 0.075 manganese-0.005 to 0.07 silicon0.06 max. sulfur-'0.01 to 0.02 phosphorus-0.01 max. oxygen--0.01 max. iron-balance.

References Cited UNITED STATES PATENTS 3,522,110 7/1970 Shimizu et al. 14812 2,878,151 3/1959 Beall et al. 148-12 3,183,078 5/1965 Ohtake et al. -49 3,239,390 3/1966- Matsukura et al. 148--12.1 3,492,173 1/1970 Goodenow 148-12 3,496,032 2/1970 Shimizu et al. 148-12 3,513,036 5/ 1970 McFarland 148-12 WAYLAND W. STALLARD, Primary Examiner US. Cl. X.R. 75-123 R; l4836 Y UNITED STATES PATENT OFFICE CERTEMCATE OF CO RECEION Patent No. 3,709,744 Dated January 9, 1973 lnv t STEPHEN R. GOODMAN, ET. AL.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 61, "In (w w v' should read 1n (W /W Column 2, line 23, "anistropy" should read anisotropy -f-w- Column 5, Table II, under Steel after F should be canceled.

Signed and sealed this 20th day of November 1973.

(SEAL) Attst EDWARD M.FLETCHER,JRT. 5 RENE D. 'IEGTMEYER Attesting Officer 1 Acting Commissioner of Patents FORM PO-1050 (10-69) uscoMM-Dc scan-P09 fi U.S, GOVERNMENT PRINTING OFFICE: I969 0-356-334. 

